1. Technical Field
This invention relates generally to the ultrasonic-nondestructive evaluation of materials, and more particularly to the ultrasonic-nondestructive evaluation of materials having a thickness of one millimeter or less.
2. Description of the Related Art
The ability of sound to propagate through a solid material, without affecting the material in any way, makes it an ideal nondestructive testing tool. By way of example, for many, many years the integrity of earthen pots has been tested by banging the pots with a stone and observing the changes in the tone of the sound produced. Through the use of modern electronics, this primitive testing technique has been modified and refined. The stone has been replaced with a piezoelelectric transmitting transducer, and the ear has been replaced with a piezoelectric receiving transducer.
The wavelength of a signal imparted into a material is inversely proportional to the frequency of the signal, so at higher frequencies the wavelength is very small in comparison to the overall dimensions of the material being tested. When the wavelength is very small in comparison to the size of the material, the size of the material does not affect the sound waves as they travel through the material. Therefore, signals in the ultrasonic frequency range (greater than 20 kHZ) are used to test various parameters of a wide variety of materials.
In non-dispersive isotropic medium, such as air, the speed of sound can be accurately measured using the well known "time of flight" method. Using the time of flight method, sound waves are injected into the air at one point and received at a distant point. The time that it takes the sound to travel from the first point to the distant point is recorded and the speed of sound calculated. In a non-dispersive isotropic medium, the phase velocity and the group velocity are identical. However, if the material is dispersive, then the time of flight method does not deliver accurate results. In order for the wavespeed in this type of material specimen to be measured, a "tone burst" method is used. Using the tone burst method, a burst of monotonic sound, typically of about ten cycles in duration, is injected into the specimen. FIG. 1 illustrates a typical measurement arrangement where a material specimen 100 is placed within an elastic medium 102, such as water, and ultrasonic energy, in this case a tone burst, is injected into the specimen along ray 1. The front face of the specimen is placed at a pre-selected coordinate, x= a, and, therefore, the back face of the specimen is located at coordinate x=b. The thickness h of the specimen 100 is defined as h=b-a.
When the incident wave (ray 1) strikes the material specimen 100 of thickness h, it is partially reflected as shown by ray 2, and partially transmitted as shown by ray 3. The wave illustrated by ray 3 is partially reflected by the back surface of the specimen 100 as shown by ray 5, and partially transmitted as shown by ray 4. This process continues indefinitely as shown by the successive rays 6-20. The time interval t between the first two reflected waves (rays 2 and 6) or between the first two transmitted waves (rays 4 and 8) is measured, and the wavespeed calculated using the equation: EQU c=2h/t, (1)
where c is the wavespeed, h is the thickness of the material specimen and t is the measured time. Attenuation is measured by fitting an exponential curve through the amplitudes of wave 2, 6, 10, etc.
While this method works well when the material specimen is sufficiently thick so that the signals are separable in the time-domain, as shown in FIGS. 2a and 2b, it cannot be used for thin specimens where the signal are not separable in the time-domain, as shown in FIGS. 2c-2e. In FIG. 2a, the first pulse 30 is the measured signal corresponding to the first reflected wave (ray 2), the pulse 32 is the measured signal corresponding to the second reflected wave (ray 6), the pulse 34 is the measured signal corresponding to the third reflected wave (ray 10), the pulse 36 is the measured signal corresponding to the fourth reflected wave (ray 14), and the pulse 38 is the measured signal corresponding to the fifth reflected wave (ray 18). Clearly, a starting point for each of these pulses can be chosen so that the distance between selected pulses may be measured.
However, since the signals are of a finite duration, when the material specimen is thin, the second reflected wave (ray 6) arrives before the first reflected wave (ray 2) has died out, and, similarly, the third reflected wave (ray 10) arrives before the second reflected wave (ray 6) has died out, and so on. Hence, a continuous arrival of signals is obtained as shown in FIGS. 2c-e. The measured signals from the reflected waves or from the transmitted waves cannot be separated so the time between the pulses cannot be measured. The time of flight method and the tone burst method cannot be used to accurately evaluate thin material specimens, but can only be applied to material specimens which have a thickness sufficient to allow the reflections from the faces of the specimen to be clearly separated in the time-domain. Therefore, the thickness of the material specimen should be large compared to the wavelength of the ultrasonic wave .lambda. injected into the specimen. For homogeneous materials, the thickness should be greater than approximately 3.lambda., and for heterogeneous materials, the thickness of the material specimen should be greater than approximately 5.lambda.. Relatively thicker specimens may be needed for highly dispersive materials such as composites.
Since wavelength and frequency are inversely proportional, attempts to solve the problem of measuring wavespeed in thin specimens have concentrated on reducing the wavelength of the ultrasonic wave by increasing its frequency. This relationship is shown by the following equation: EQU c=.function..lambda. (2)
where c=wavespeed, .function.=frequency and .lambda.=wavelength. For example, using equations (1) and (2), to determine the wavespeed in a one millimeter thick aluminum plate, one would need to use a frequency of approximately 15 M Hz since the wavespeed in aluminum is 6.38 mm/.mu.s. Likewise, in order to determine the wavespeed in a specimen having a thickness of 0.1 millimeters, one would need to use a frequency of 150 MHz. Practically, this means that transducers capable of emitting acoustical energy at 15 MHz and at 150 MHz, respectively, must be used. This is a major disadvantage for the previous mentioned techniques, because 15 MHz transducers are extremely expensive, as compared to 1 MHz transducers for instance, and 150 MHz transducers are not readily available, commercially. Not only is the cost of the transducers prohibitive, but the cost of the associated electronics which are capable of accurately delivering and processing these extremely high frequencies increases exponentially with the frequency. Because of these problems, these ultrasonic non-destructive evaluation techniques are facing a wavelength barrier where the thickness of a material specimen to be evaluated must be several times greater than the wavelength.
In addition to the two techniques already mentioned, many other ultrasonic non-destructive evaluation techniques are known, however, they are all limited by of the thickness of the material specimen due to the use of the pulse separation as shown in FIGS. 2a and 2b. Refinements of the time-of-flight method include the pulse-superposition method and the pulse-echo method. Using these measurement techniques, the specimens have to be thick enough so that individual pulses can be distinguished in the time-domain. Moreover, if the specimen material is highly attenuating, then the pulse-superposition method cannot be used, and if the specimen material is dispersive, then the pulse-echo method gives unreliable results.
The "ultrasonic spectroscopy" method was developed for measuring wavespeed in thin composite laminates. The reflected pulse from the front face of the laminate and the reflected pulse from the rear face of the laminate are digitized and transformed into the frequency-domain by a Fast Fourier Transform. Interference between these two reflected pulses produces resonance dips in the amplitude of the transformed signals, and the spacing between these dips can be related to wavespeed. Using this technique, wavespeed can be measured in specimens of about 2 millimeters in thickness over a frequency range of 5 to 11 M Hz. The limitations of this method are: (a) it requires pulse separation; (b) it is not conducive to automation; (c) in materials having high attenuation, the resonance dips are not sharp, and, thus, the method is prone to errors; and (d) when the spacing between the resonance dips is large, the method yields very inaccurate results.
It should be remembered that wavespeed and attenuation of a material specimen can be used to determine many parameters of the material specimen. For instance, if the wavespeed in a particular type of material is known and if the time that it takes for the ultrasonic wave to pass through the material can be measured, then the thickness of the material can be determined, as shown by equation (1). If the thickness of the material specimen is known, then an elastic modulus of the material can be determined. For instance, if the wavespeed and the density of the material is known, then the stiffness of the material can be determined.
It is important to reiterate that ultrasonics were originally used to determine the integrity of a material or structure. The ultrasonics eliminate the effect of the size of the structure, and, therefore, any scattering of the ultrasonic wave is due to a flaw in the structure. Ultrasound has been used to detect cracks, holes, porosity and non-homogeneity in isotropic materials. Basically, an ultrasonic wave is launched into a specimen, and the sound is scattered from the defects and detected by a receiving transducer. The depth of a defect can be estimated by measuring the time of flight from the emitter to the receiver, and the extent of damage can be mapped by moving the transducer over the damage and observing the reflections. The reflections from other surfaces, such as the back surface of the specimen, can be gated out and reflections from the defect isolated.
It is well known that the wavespeed of sound in an elastic material is related to its stiffness, E=c.sup.2 p, where E=stiffness, c=the wavespeed, and p=density. The presence of defects, such as voids, cracks, particles, and delaminations, changes the effective stiffness of the material. When an acoustical wave has propagated through such a material, the change in stiffness is manifested as a change in the sound velocity as shown in the previous equation. Furthermore, the defects tend to scatter the sound waves, and, as a result, the defect population also produces attenuation of the wave as it passes through the material.
Various researchers have used ultrasonics for the non-destructive evaluation of the composites by relating the acoustic parameters of the composites to the damage or defects in the composite. The transmission of ultrasonic energy through a composite material in the direction of its thickness has been used for flaw detection in metals. If there are no cracks in the specimen, the waves pass through undisturbed. When the waves encounter cracks, however, the energy is scattered by the cracks, and, as a result, wave intensity is reduced.
Composites are finding an ever increasing use as structural material, especially in the aerospace and automotive industries. In the early stages of the development of composites, composites were used as secondary structural members such as control surface panels. As the reliability of composites increased and material with higher strength-to-weight ratios have been produced, composites are now being used as primary load bearing members. Therefore, information regarding the integrity of composites such as these is growing more useful and often vital.
The mechanical and thermal loading cycles that the structures have to undergo create damage in the composite structures. Wile a wide variety of nondestructive evaluation techniques which were originally developed for homogeneous materials have been used on composite materials with some success, the damage development in composites is quite different from the damage development in isotropic materials. In isotropic materials when the damage is initiated, it becomes the nucleating sight for further damage growth. Any further loading causes the stress concentration around the damage to produce greater damage. The growth mechanism and growth rates for isotropic homogeneous materials are fairly well understood, and hence, the main task for the nondestructive evaluation of such materials is to detect and record the damage as early as possible in its growth life. Once this information is available, reasonable prediction of the residual stiffness and of the remaining life of the component can be determined.
Conversely, in composite materials, and especially in continuous fiber composites, a very different phenomenon takes place. In composites, very strong fibers, such as graphite fibers, are embedded in a weaker matrix, such as epoxy. As a result, matrix cracking is usually the first mode of damage in composites because the matrix is not nearly as strong as the fibers. Fortunately, however, the fibers help contain the matrix cracks in two ways: (1) fibers inhibit the crack growth by acting as crack arresters; and (2) fibers, being much stronger than the matrix, are able to carry the extra load due to the load redistribution of stresses in the vicinity of the damage, and, thus, some amount of stress relieving takes place in the matrix. Therefore, once a composite has been damaged, further loading of the composite causes the next crack in the matrix to occur at a different location where the matrix stress has reached a critical value. As a result of this phenomenon, the entire composite structure develops micro-cracks without seriously endangering the overall integrity of the structure.
The immediate effect of this distributed damage is in the form of reduced stiffness and higher damping of the structure. Continued damage to the composite structure causes the response of the structure to change under load. For example, the reduced stiffness of the wing of an aircraft will cause extra deflection and possibly torsion, both of which may change the aerodynamic configuration of the wing. The evaluation of a composite structure after it has undergone a certain amount of loading and damage is useful in evaluating the performance of the structure. Therefore, the interest is not only the detection and location of a defect, but also the evaluation of the effects of continued microdamage on the mechanical response of a component. Almost all of the non-destructive evaluation techniques listed earlier can detect macrosize flaws, but cannot measure the changes in mechanical response of the materials caused by distributed damage.
Another area of interest is the testing of adhesive bond strength, particularly for in-situ applications. However, there does not exist any reliable method for the non-destructive evaluation of adhesive interfaces. This, again, is due to the fact that the wavelength of an ultrasonic wave is much larger than the bond-line thickness of an adhesive interface. Since the previously mentioned methods of ultrasonic non-destructive evaluation rely on the separation of successive pulses in the time-domain, these methods are not useful in the evaluation of thin adhesive bonds. Since adhesives are often used in critical structural applications, such as the adhesion of ceramic tiles to the United States space shuttles to protect them against the heat of re-entry, a method of non-destructive evaluation which will determine the integrity of the adhesive bonds is badly needed.